Over 2,000 customers of Tesco - the U.K.'s biggest supermarket - have had their personal details posted online after a data breach at the company by computer hackers.
Tesco has now deactivated some customers' net accounts after login names and passwords were shared online on a popular text-sharing website, according to a report by the BBC.
Hackers used data stolen in other high-profile security breaches to draw up the list of Tesco.com details and found that more than 2,000 customers had used the same email and password combinations on the other sites, according to the reports.
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Tesco told CNBC that it takes the security of its customers' data extremely seriously and is urgently investigating these claims.
"We are contacting all customers who may have been affected and are committed to ensuring that none of them miss out as a result of this. We will issue replacement vouchers to the very small number who are affected," a spokesperson said in an e-mailed statement on Friday morning.
Investors did not appear concerned by the data breach on Friday morning, with shares of the U.K. grocer just tipping into the red with a loss of 0.1 percent. This compares to a slip of 2.2 percent for U.S. retailer Target on the day it announced that as many as 40 million credit and debit cards ad been compromised.
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On December 19, Target said the breach had been caused by malware installed on the company's point of sale registers. The store added a few days after that an investigation had found that at least 70 million customers' personal information was stolen from its database —including names, mailing addresses, telephone numbers, and email addresses. Some victims did not shop at Target during the time of the breach, said the retailer.
Ishaq Siddiqi, a market strategist at U.K.-based broker ETX Capital believes that investors have greater concerns surrounding Tesco's stock than the data breach, with declining sales hitting the supermarket's recent financial updates.
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"That's not saying this data breach will go unnoticed; I would assume that management now have to launch a thorough review of internal data security systems, audits and improving their security protocols to move past this episode," he told CNBC via email.
The headline on this story has been changed since it was first published to reflect that Tesco's hacking attack appears different to what has happened to Target in the US.
—By CNBC.com's Matt Clinch; Follow him on Twitter